Many types of devices make use of batteries which have multiple cells. In the field of consumer electronic devices, items such as tablets or notebook computers will typically have battery packs that comprise multiple cells.
DC-DC conversion is required to either step down or step up the battery voltage as the supply voltage of the battery pack varies during battery discharge.
Where multiple cells are provided in series, the voltage supplied by a fully charged battery will be higher than the voltage required for operation of the device and so a step down DC-DC conversion is required. A buck converter is usually used for this purpose, which uses an inductor as an energy storage element and has switching elements that selectively couple the inductor with an input voltage, in order to step down the voltage.
However, stepping down from a relatively higher voltage with a buck converter requires relatively high inductance or a relatively high switching frequency and so there are either large components or large losses within the buck converter. In general, stepping up or down a large voltage with a DC-DC converter involves large switching losses.
Multi-level DC-DC converters address this problem by inserting one or more switching capacitors to provide one or more intermediate voltage levels between the input and output voltages.
However, the output current is always provided with two or more switches in series which reduces efficiency, especially for high loads.